1. Field
Embodiments of the disclosure relate generally to the field of rotorcraft and more particularly to embodiments for a retractable disc rotor system employing a two stage retraction system with integrated screw and hydraulic actuation.
2. Background
Efficient operation of helicopters as vertical takeoff or landing (VTOL) aircraft is limited typically by cruise speed which cannot exceed a critical advancing and receding rotor speed. Contra-rotating rotors and other approaches have been used to reduce this requirement however systems employing fixed wing lift for cruise are desirable. Disc rotor systems which allow partial or full retraction of the rotor blades have been proposed to allow conversion to wing supported flight for cruise thereby allowing higher speeds. Existing solutions and ideas are undeveloped and use a single device such a wire cable or ball screw for retraction of rotor blades. Such prior art designs may also lack or require complex supplemental safety mechanisms designed to prevent failures. A cable mechanism requires a relatively heavy cable to support the high centrifugal loads imposed on the rotor blades. It also requires a large capstan, gear reductions and idler pulleys to achieve the high force required to retract a blade. Cables are subject to relatively high stretch under load which may make synchronizing retraction positions from one blade to another difficult. When taking the high centrifugal loads into account, a device using only a ball screw may be unreliable and short lived due to the need to make it small enough to fit inside a rotor blade. If the device powering the ball-screw should fail, the screw can be backdriven unless it has a brake large enough to counter loads from centrifugal force. Back-driving could result in an uncommanded extension of a single blade resulting in an imbalance of the rotor. A device similar to the ball-screw, a dedicated hydraulic cylinder, must also be small enough to fit inside the rotor blade. This small size requires hydraulic pressures in the range of 5,000 psi to be useful. These high pressures may be difficult to seal in a flexible assembly.
It is therefore desirable to provide a disc rotor retraction system which eliminates blade retraction synchronization issues and uncommanded extension while providing a compact arrangement which may be accommodated within the rotor blade cord.